Electroactive and Photoactive Rod-Coil Block Copolymers: Self-Organization and Photophysical Properties

Two series of new electroactive and photoactive coil-rod-coil and rod-coil-rod triblock copolymers, poly (pentadecamethylene carboxester)-block poly(p-phenylene benzobisthiazole) block poly (pentadecamethylene carboxester) (1), and poly(2,6-benzoxazole)-Woc£poly(benzobisthiazole decamethylene)-WocÄ:-pory(2,6-benzoxazole) (2), were synthesized, characterized, and used to investigate the self-assembly properties of rod-coil block copolymers' The progressive band narrowing of the absorption spectrum of thin films of 1 confirmed the effects of spatial confinement with increasing coil block size. Photoluminescence studies of thin films of 1 and 2 showed the effects of self-organization, annealing at 110 °C, block lengths, and composition on photophysical properties. Bilayer photoreceptors consisting of a layer of block copolymer as the charge generation layer and a layer of tris(/?-tolyl)amine dispersed in polycarbonate as a trap-free hole transport layer were oberved to have high quantum efficiency, good photosensitivity and good dark decay. INTRODUCTION Rod-coil block copolymers are currently of theoretical [1-4] and experimental [5-7] interest because of their complex morphologies and phase behavior. Because of the thermodynamic incompatibility of the two vastly different conformations of rigid-rod and coillike blocks and the constraint imposed by the chemical bonding between the blocks, rod-coil block copolymers exhibit a wide range of microphase-separated morphologies. The rich variety of ordered morphologies in this class of block copolymers are being explored and the factors determining their phase behavior are being defined [1-7]. However, rod-coil block copolymers which have been studied so far are non-electroactive and non-photoactive[5-7], thus limiting both the scope of the techniques suitable for investigating the morphologies and applications of the resulting nanophase materials. In this paper, two series of new electroactive and photoactive rod-coil-rod and coil-rodcoil triblock copolymers: poly (pentadecamethylene carboxester)-£/odt poly(p-phenylene benzobisthiazole) block poly (pentadecamethylene carboxester) (1, CRCA) and poly(2 6benzoxazole)-&/oc£-poly(benzobisthiazole decamethylene)-Wocfc-poly(2,6-benzoxazole) (2 RCRA-1) have been synthesized, characterized, and used to explore the self-assembly properties of block copolymers and the effects of the morphologies of the resulting self-organized nanostructured materials on their solid-state photophysical properties. The chemical structures of the tnb ock copolymers we investigated are shown below. The poly(p-phenylene benzobisthiazole) (3, PBZT) homopolymer, is a well-known conjugated rigid-rod polymer which has interesting photoconductive [8] and light emitting [9] properties. The coillike blocks of 1 consist of blocks of various lengths of the nonphotoactive and nonelectroactive polyester 4 The poly(2 6-benzoxazole) (5, 2,6-PBO) homopolymer is a conjugated polymer with high modulus and thermal stability [10], that also exhibits liquid crystalline ordered phases in solution [11] However, its electroactive and photoactive properties have not been reported before The po y(benzobisthiazole decamethylene) (6, PBTC10) is a non-photoactive and non-electroactive polymer, soluble m common organic solvent such as THF, chloroform. Various techniques such as differential scanning calorimetry, polarized optical microscopy, optical absorption and photoluminescence spectroscopies, and cyclic voltammetry, were used to probe and elucidate the morphologies and properties of the block copolymers. Absorption and photoluminescence measurements on the thin films were performed, and confirmed the self-organizaton of the rigidcoil block copolymers. -fn2cA-0-C v '15 tfo-COO \ / rO C-O-fCH^ m '15 1.CRCA 1a:m=9, n=2; 1d: m=9, n=9 1b: m=9, n=3; 1e: m=9,n=19 1c: m=9, n=4 ta:^:xxiH-iH<:jor L J20 |_ J20 20 2, RCRA-1 /-\ß-Y^r:XJQ S 3, PBZT o -C-0-fCH2 m /15J 4, Polyester N< CH2 N k 5, 2,6-PBO 6, PBTC10 Figure 1. Chemical structures of 1 and 2 copolymers and their related homopolymers. EXPERIMENTS The coil-rod-coil triblock copolymers la-le were synthesized by copolymerization of a carboxylic acid-terminated PBZT (HOOC-PBZT-COOH) [11] with the AB-type monomer 16hydroxyhexadecanoic acid (16-HA). The resulting products of copolymerization were purified by extensive extraction with refluxing acetone, which is a selective solvent for the polyester. Triblock copolymer 2, was synthesized according to modified literature method [11]. In short, carboxylic acid-terminated flexible poly(benzobisthiazole decamethylene) (PBTC10) block (HOOC-Bm-COOH) was synthesized by reacting 2,5-diamino-l,4-benzenedithiol dihydrochloride (DABDT) with excess 1,10-decanedicarboxylic acid in polyphosphoric acid (PPA). Then, 4amino-3-hydroxybenzoic acid (A) was added to generate the rigid-rod blocks. The rod-coil-rod triblock sample we investigated here is A20B20A20, with average numbers of repeat units of 20 for 2,6-PBO and PBTC10 repeat unit respectively. Optical quality thin films of 1, 2, and their blends with PMMA were obtained by spin coating onto silica substrates from nitrobenzene(NB)/GaCl3 solution. The films were then regenerated in methanol and dried in a vacuum oven. The bilayer receptors for the photodischarge measurements were prepared by spin coating the solution of a copolymer onto polyethylene terephthalate) (PET) substrates, which were pre-coated with nickel. The copolymer films were then coated with a 20-mm tris(p-tolyl)amine/polycarbonate (TTA:PC 40/60, w/w) layer Photophysical measurements, such as, UV-Vis, steady-state photoluminescence(PL), and charge photogeneration, were all performed on thin films at room temperature. Details of instruments and methods have been previously reported [8,9]. RESULTS AND DISCUSSION The structures and compositions of the triblock copolymers la-le and 2 were established by various techniques, including 'H NMR,C NMR, FTIR, DSC, TGA, UV-Vis, and photoluminescence spectroscopies. The expected microphase separation or self-assembly processes of rod-coil triblock copolymers are illustrated in Figure 2. In isotropic solution, the triblock copolymers are in a disorder state. In the solid state, however, phase-separated ordered structures in which the rodlike blocks aggregate into anisotropic domains are the thermodynamically stable structures [1-7]. Thus, the self-assembly process is a kinetic process that occurs during the coagulation of solutions into solids as well as during subsequent processing (e.g. annealing) of the solids. Because the rodlike blocks are electroactive and photoactive, optical techniques can be used as powerful probes of the self-assembly process and the resulting nanostructures. Figure 2. Schematic illustration of self-assembled structures from coil-rod-coil and rodcoil-rod triblock copolymers.